The Irish Mythological Cycle are those stories which pretain to the settling of Ireland and the adventures of its gods, the Tuatha De Danann. Their story begins in the Book of Invasions, a long pseudohistorical text which details five successive conquerings of the island until the coming of the de Dannan, and their subsequent overthrow by the Milesians. It also tells of how the de Dannan, lead by the god of lightning, Lugh Lamhfada, defeat the forces of darkness--the Fomorians--and bring agriculture and art to the island.

Subsequent stories tell of the gods' days of glory living above ground--such as "The Second Battle of magh Turedh" which gives longer details into events described in The Book of Invasions, or the revenge story of "The Children of Tureen"--the three of which form the basis of the first "half" of the mythological cycle, the settlement and loss of Ireland.

The second "half"--though I don't believe anyone has designated it as such--details what happens to the gods once the Milesians--the modern Celts--have invaded and driven them to live underground in the hollow hills. First is the tragic story of "The Children of Lir" which tells of how the chosing of a new king for the de Dannan leads to Lir's children being turned into swans for 900 years.

There is then the three stories of "The Dream of Oengus" "The Wooing of Etain" and "The Destruction of Da Derga's Hostel"--each building on the other into a story of lost love, betrayal, and the breaking of geasa. It is essentially how the de Dannan get revenge on the tretchery of the Milesians, who have stolen their island and inadvertanly steal one of their wives--Etain. This sets off a chain of events culminating in the death of King Conaire Mor, and sets a prelude to the Ulster Cycle.

The stories are found in most of the Irish manuscripts, particularly in The Book of Leinster, the Book of Fermoy, the Book of Ballymote, and The Book of the Dun Cow. Except for "The Fate of the Children of Lir" which exists in a much later (17th century) manuscript. The Mythological cycle, however, was the last to take on shape, and as such betrays a much more "Dark Ages" atmostphere than Bronze age Ireland, the way that the Ulster Cycle does, or an almost Neolithic atmosphere, like the Fionn Cycle. It is ironic that the setting of each cycle is inverse with its age, excepting the Cycle of the Kings and the Saints.
The Stories

The Book of Invasions

The Second Battle of Magh Turedh

The Fate of the Children of Tureen

The Fate of the Children of Lir

The Dream of Oengus

The Wooing of Etain

The Destruction of Da Derga's Hostel

The Fosterage of the House of Two Milk-pails

The Story of Tuan mac Carill

Most of the stories are either online in some form, or can be found in either Ancient Irish Tales translated by Cross & Slover, or The Encyclopedia of Celtic Wisdom edited by Caitlin and John Matthews.